To perform position location in cellular networks (e.g., a cellular telephone network), several approaches perform triangulation based upon the use of timing information sent between each of several basestations and a mobile device, such as a cellular telephone. In one approach, called Time Difference of Arrival (TDOA), the times of reception of a signal from a mobile device is measured at several basestations, and these times are transmitted to a location determination entity, called a location server, which computes the position of the mobile device using these times of reception. For this approach to work, the accurate positions of the basestations need to be known, and the times-of-day at these basestations need to be coordinated in order to provide an accurate measurement of the location. Time coordination is an operation to keep, at a particular instance of time, the times of day associated with multiple basestations within a specified error bound.
FIG. 1 shows an example of a TDOA system where the times of reception (TR1, TR2, and TR3) of the same signal from the mobile cellular telephone 111 are measured at cellular basestations 101, 103, and 105 and processed by a location server 115. The location server 115 is coupled to receive data from the basestations through the mobile switching center 113. The mobile switching center 113 provides signals (e.g., voice communications) to and from the land-line Public Switched Telephone System (PSTS) so that signals may be conveyed to and from the mobile telephone to other telephones (e.g., land-line phones on the PSTS or other mobile telephones). In some cases the location server may also communicate with the mobile switching center via a cellular link. The location server may also monitor emissions from several of the basestations in an effort to determine the relative timing of these emissions.
An alternative method, called Enhanced Observed Time Difference (EOTD) or Advanced Forward Link Trilateration (AFLT), measures at the mobile device the times of arrival of signals transmitted from each of several basestations. FIG. 1 applies to this case if the arrows of TR1, TR2, and TR3 are reversed. This timing data may then be used to compute the position of the mobile device. Such computation may be done at the mobile device itself, or at a location server if the timing information so obtained by the mobile device is transmitted to the location server via a communication link. Again, the times-of-day of the basestations must be coordinated, and their locations accurately assessed. In either approach, the locations of the basestations may be determined by standard surveying methods and be stored in the basestations, at the location server, or elsewhere in the network in some type of computer memory.
Yet a third method of doing position location involves the use in the mobile device of a receiver for the Global Positioning Satellite System (GPS) or other satellite positioning system (SPS). Such a method may be completely autonomous or may utilize the cellular network to provide assistance data or to share in the position calculation. Examples of such a method are described in U.S. Pat. No. 5,841,396; No. 5,945,944; and No. 5,812,087. As a shorthand, we call these various methods “SPS”. In practical low-cost implementations, both the mobile cellular communications receiver and the SPS receiver are integrated into the same enclosure and, may in fact share common electronic circuitry.
A combination of either the EOTD or TDOA with an SPS system is called a “hybrid” system.
It should be clear from the above description that, for EOTD, TDOA, or hybrid systems, time coordination between the various cellular basestations is necessary for accurate position calculation of the mobile device. The required accuracy of the times-of-day at the basestations depends upon the details of the positioning method utilized.
In yet another variation of the above methods, the round trip delay (RTD) is found for signals that are sent from the basestation to the mobile device and then are returned. In a similar, but alternative, method the round trip delay is found for signals that are sent from the mobile device to the basestation and then returned. Each of these round-trip delays is divided by two to determine an estimate of the one-way time delay. Knowledge of the location of the basestation, plus a one-way delay constrains the location of the mobile device to a circle on the earth. Two such measurements then result in the intersection of two circles, which in turn constrains the location to two points on the earth. A third measurement (even an angle of arrival or cell sector) resolves the ambiguity. With the round trip delay approach, it is important that the RTD measurements be coordinated to be taken within several seconds, at worst, so that if the mobile device is moving rapidly, the measurements correspond to the mobile device being near the same location.
In many situations, it is not possible to perform round trip measurements to each of two or three basestations, but only to one basestation, which is the primary one communicating with the mobile device. For example, this is the case when the IS-95 North American CDMA cellular standard is used. Or it may not be possible to perform accurate (e.g., sub microsecond) round trip timing measurements at all due to equipment or signaling protocol limitations. This appears to be the case when the GSM cellular communication standard is used. In these cases, it is even more important that accurate timing (or relative timing) be maintained on the basestation transmissions if a triangulation operation is to be performed, since only the time differences between different mobile-basestation paths are utilized.
Another reason to maintain accurate timing information at basestations is to provide time to the mobile devices for aiding GPS based position calculations; and such information may result in reduced time to first fix, and/or improved sensitivity. U.S. Pat. No. 6,150,980 and No. 6,052,081 contain such examples. The required accuracy for these situations can range from a few microseconds to around 10 milliseconds, depending upon the performance improvement desired. In a hybrid system, the basestation timing serves the dual purpose of improving the TDOA (or EOTD) operation as well as the GPS operation.
The prior art approaches to basestation timing coordination employ special fixed location timing systems, termed Location Measurement Units (LMU) or Timing Measurement Units (TMU). These units typically include fixed location GPS receivers which enable the determination of accurate time-of-day. The location of the units may be surveyed, such as may be done with GPS based surveying equipment. In alternative implementations, the LMUs or TMUs may not rely upon an absolute time provided by a GPS receiver or other source, but may simply relate the timing of one basestation versus that of another basestation, in a differential sense. However, such an alternative approach (without using a GPS receiver) relies upon the observability of multiple basestations by a single entity. Furthermore, such an approach may give rise to cumulative errors across a network.
Typically, LMUs or TMUs observe the timing signals, such as framing markers, present within the cellular communication signals that are transmitted from the basestations and attempt to time-tag these timing signals with the local time found via a GPS set or other time determination device. Messages may subsequently be sent to the basestations (or other infrastructure components), which allow these entities to keep track of elapsed time. Then, upon command, or periodically, special messages may be sent over the cellular network to mobile devices served by the network indicating the time-of-day associated with the framing structure of the signal. This is particularly easy for a system such as GSM in which the total framing structure lasts over a period exceeding 3 hours. Note that the location measurement units may serve other purposes, such as acting as the location servers—that is, the LMUs may actually perform the time-of-arrival measurements from the mobile devices in order to determine the position of the mobile devices.
One problem with these LMU or TMU approach is that they require the construction of new special fixed equipment at each basestation or at other sites within communication range of several basestations. This can lead to very high costs for installation and maintenance.